N-P doped lignin flame retardant: preparation and application for thermoplastic polyurethane

A nitrogen‑phosphorus co-doped lignin-based flame retardant (L-N-Zn-ATMP) was successfully synthesized and incorporated into thermoplastic polyurethane (TPU) to enhance its flame retardancy. L-N-Zn-ATMP contained abundant benzene rings, alkoxy, and phosphate groups. Incorporation of this additive significantly improved the residual char rate of TPU composites, which increased from 0.19 % (pure TPU) to 7.52 %. Compared with pure TPU, the peak heat release rate and peak smoke production rate of TPU/10phr L-N-Zn-ATMP were reduced by 55.86 % and 50.00 %, respectively. The flame-retardant mechanism was proposed based on the comprehensive analysis of the residual char's structure and composition and thermal decomposition products. The synergistic effect between nitrogen and phosphorus promoted TPU carbonization and enhanced char formation, thereby providing thermal insulation and oxygen barrier effects. The non-combustible gases generated from L-N-Zn-ATMP effectively diluted the concentration of flammable volatiles generated during combustion. Although the TPU composites showed a slight decline in mechanical properties, they still met the practical application requirements. This study provided insights into the design of high-performance lignin-based flame retardants.